Elastomeric (rubbery) latex products having an optimized distribution of large and small particles interdispersed therein are desirable for many utilizations and applications.
Particularly desirable uses for such materials include the so-called "rubber-modified" and/or "rubber-reinforced" polymer compositions whereby the impact resistance of various polymeric compositions is improved to provide desirable resins and resin products. These may be merely blended polymer (i.e., "polyblend" ) compositions or grafted polyblend compositions made by polymerizing a usually addition-polymerizable monomer (or mixture of monomers), such as styrene or a suitable mixture of styrene and acrylonitrile, in the presence of a dissolved or otherwise dispersed elastomer, such as polybutadiene to achieve a matrix comprising some polymer grafted to said elastomer and some that is not. Among the many desirable resin materials so preparable are: expandable or foam-providing compositions; molding materials; coatings and so forth.
The physical properties of rubber-reinforced or -modified polymeric products are frequently materially dependent upon the degree and variety of the particular dispersion of the elastomeric modifying agent to be found within the polymer matrix.
Predominant among the effects of having included in the polymeric matrix, elastomeric latices and various other rubber particle inclusions of relatively small average particle size, such as those having approximately 1,000 Angstrom (A) or so particle diameters, is the tendency of such rubber-containing compositions to result on shaping, as in injection molding operations, in formed articles having high gloss. At the same time, the small particle size can sometimes lead to intolerably low impact resistance and detrimental effects on other physical properties such as elongation. On the other hand, the obverse is frequently true (e.g., low gloss--high impact resistance, etc.) when the particle size of the rubber inclusion is relatively large, such as those with an average particle size diameter in the range of 1-5 or more microns (.mu.).
It is well known in the art that the more attractive and desirable rubber-modified polymer compositions for most molding and other purposes achieve and exhibit optimized compromise in properties when they have therein interdispersed a distribution of relatively large through relatively small elastomer particles, such a distribution being hereinafter referred to as a polydisperse particle size distribution. Notwithstanding, effectively, efficiently and economically achieving optimum particle size distribution is not easily accomplished.
For example, obtaining polybutadiene and like elastomeric latex products having particles of relatively larger size therein by polymerization to such sizes usually necessitates resorting to undesirably and inconveniently long reaction times for their preparation. A further problem associated with the relatively small sized particles is that polymerization techniques, per se, do not always avoid the disadvantages of having to handle (usually in somewhat bulky, low solids level intermediate forms) latex materials which contain relatively small-sized elastomer particles in a narrow distribution range, such as is often found in latex products wherein average particle size diameter is on the order of about 800 to about 1200 A.
One general technique that, by means of various approaches to its accomplishment, has had application in order to generate larger-sized particles especially in polydispersion with smaller-sized particles in elastomer latex products is to induce limited or controlled agglomeration of at least a portion of the smaller particles in the latex material being treated. This has been done by employment of various agglomerating agents including some that are polymeric in nature. In any such procedure, it is oftentimes important to avoid actual coagulation of the latex undergoing the agglomeration treatment. This is especially true when the elastomer produced, having polydispersed particle size is intended for subsequent direct use as unrecovered elastomer-containing emulsion or dispersion in reactive processing to make graft-polymerized products with styrenic polymers, particularly copolymers of styrene and acrylonitrile. Of course, precautions against coagulation are without bearing if the polydisperse-particle-size elastomer in an agglomerated latex is to be recovered as "crumb" (or the like) (a) for direct manufacture into rubber goods, (b) for polyblending operations or (c) for ensuing dissolution in mass (or bulk) or other solvent-system preparations of various graft copolymerized products.
There have been several noteworthy procedures to effect limited or controlled agglomeration in elastomeric latices. These include such approaches as are disclosed in:
(a) U.S. Pat. No. 3,666,704 to Keppler and Wesslau wherein it is taught that some rubber latices, namely those which are less than 95% polymerized can be agglomerated to yield a latex having larger particles by employment of an agglomerating agent which is a latex of a polymer containing polymerized therein from 0.5 to 50 weight percent of a monomer which yields a water soluble polymer.
(b) U.S. Pat. No. 3,956,218, also to Keppler and Wesslau, wherein a synthetic rubber latex is similarly agglomerated by employment of an agglomerating agent which is a latex of a polymer containing polymerized therein a minor amount of monomer which yields a water-soluble polymer, which polymeric latex agglomerating agent also contains polymerized throughout a major amount of a monomer which yields a water-insoluble polymer.
(c) U.S. Pat. No. 3,825,621 to Ford, wherein a process is taught to use an interpolymeric agglomerating agent to produce an agglomerated latex which is then used in a grafted polyblend composition.
Other teachings involving rubber-containing polymer compositions having polydisperse rubber particle size characteristics are included in U.S. Pat. Nos. 3,446,873; 3,509,237; 3,991,136 and 4,009,226; the teachings of all of which as well as those in the References identified in the foregoing items (a) through (c) are here incorporated by reference thereto.
Nonetheless, nothing in prior art appears to realistically concern itself with a simple and efficient way to achieve controlled agglomeration of elastomeric latices, including polybutadiene rubber latex products, to get better particle size polydispersion using minimized amounts of agglomerating agent and achieving minimized levels of deleterious residual contaminants.